Question: In history class, the girl to boy ratio is $2$ to $7$. If there are $8$ girls, how many boys are there?
Solution: A ratio of $2$ girls to $7$ boys means that there is a group of $2$ girls for every group of $7$ boys. If there are $8$ girls, then there are $4$ groups of $2$ girls each. Because we know the ratio is $2:7$ , there must be $4$ groups of $7$ boys each. There is a total of $28$ boys in history class.